1. Field of the Invention
This invention relates generally to a premix fuel nozzle for use in a gas turbine combustor and more specifically to a premix fuel nozzle that does not contain a fuel circuit dedicated to support a pilot flame.
2. Description of Related Art
The U.S. Government has enacted requirements for lowering pollution emissions from gas turbine combustion engines, especially nitrogen oxide (NOx) and carbon monoxide CO. These emissions are of particular concern for land based gas turbine engines that are used to generate electricity since these types of engines usually operate continuously and therefore emit steady amounts of NOx and CO. A variety of measures have been taken to reduce NOx and CO emissions including the use of catalysts, burning cleaner fuels such as natural gas, and improving combustion system efficiency. One of the more significant enhancements to land based gas turbine combustion technology has been the use of premixing fuel and compressed air prior to combustion. An example of this technology is shown in FIG. 1 and discussed further in U.S. Pat. No. 4,292,801. FIG. 1 shows a dual stage dual mode combustor typically used in a gas turbine engine for generating electricity. Combustor 12 has first stage combustion chamber 25 and a second stage combustion chamber 26 interconnected by a throat region 27, as well as a plurality of diffusion type fuel nozzles 29. Depending on the mode of operation, combustion may occur in first stage combustion chamber 25, second stage combustion chamber 26, or both chambers. When combustion occurs in second chamber 26, the fuel injected from nozzles 29 mixes with air in chamber 25 prior to ignition in second chamber 26. As shown in FIG. 1, an identical fuel nozzle 29 is positioned proximate throat region 27 to aid in supporting combustion for second chamber 26. While the overall premixing effect in first chamber 25 serves to reduce NOx and CO emissions from this type combustor, further enhancements have been made to the centermost fuel nozzle since fuel and air from this fuel nozzle undergo minimal mixing prior to combustion.
A combined diffusion and premix fuel nozzle, which is shown in FIG. 2, has been used instead of the diffusion type fuel nozzle shown proximate throat region 27 in FIG. 1. Although an improvement was attained through premix nozzle 31, this nozzle still contained a fuel circuit 32 that contained fuel that did not adequately mix with air prior to combusting and therefore contributed to elevated levels of NOx and CO emissions. As a result, this fuel nozzle was modified such that all fuel that was injected into a combustor was premixed with compressed air prior to combustion to create a more homogeneous fuel/air mixture that would burn more completely and thereby result in lower emissions. This improved fully premixed fuel nozzle is shown in FIG. 3 and discussed further in U.S. Pat. No. 6,446,439. Fuel nozzle 50 contains a generally annular premix nozzle 51 having a plurality of injector holes 52 and a premix pilot nozzle 53 with a plurality of feedholes 54. In this pilot circuit embodiment, fuel enters a premix passage 55 from premix pilot nozzle 53 and mixes with air from air flow channels 56 to form a premixture. Fuel nozzle 50 is typically utilized along the centerline of a combustor similar to that shown in FIG. 1 and aids combustion in second chamber 26. Although the fully premixed fuel nozzle disclosed in FIG. 3 provides a more homogeneous fuel/air mixture prior to combustion than prior art fuel nozzles, disadvantages to the fully premixed fuel nozzle have been discovered, specifically relating to premix pilot nozzle 53. More specifically, in order to maintain emissions levels in acceptable ranges, premix pilot feed holes 54 had to be adjusted depending on the engine type, mass flow, and operating conditions. This required tedious modifications to each nozzle either during manufacturing or during assembly and flow testing, prior to installation on the engine.
In order to simplify the fuel nozzle structure and further improve emissions, it is desirable to have a fuel nozzle that supports combustion in a second combustion chamber 26 without having a pilot circuit. Elimination of a pilot circuit, whether diffusion or premix, will further reduce emissions since the pilot circuit is always in operation whether or not it was actually needed to support combustion. Furthermore, eliminating the pilot circuit will simplify fuel nozzle design and manufacturing. The major concern with eliminating the pilot circuit is combustion stability in the second combustion chamber given the reduced amount of dedicated fuel flow to the secondary fuel nozzle. Experimental testing was conducted on a gas turbine combustor having first and second combustion chambers by blocking the premix pilot nozzle 53 of fuel nozzle 50 in accordance with FIG. 3. The combustor was run through its entire range of operating conditions and positive results were obtained for maintaining a stable flame in the second combustion chamber. Changes in combustion dynamics or pressure fluctuations associated with the elimination of the pilot fuel circuit were found to be minimal and insignificant for typical operating conditions.
An additional concern with prior art fuel nozzles relates to the amount of cooling air directed to the nozzle tip. While providing air to cool the nozzle tip region is necessary to prevent damage from exposure to the elevated temperatures, too much air can adversely affect combustion dynamics. This is especially a concern for fuel nozzles not having a pilot fuel circuit.